This invention relates to a method and apparatus for controlling engine and brake torque to selectively engage a front drive axle to provide all wheel drive under optimal conditions.
Vehicles utilize all wheel drive systems to achieve improved vehicle control under poor road conditions. All wheel drives for trucks equipped with geared front axle clutched transfer cases are normally engaged and disengaged by a vehicle operator or are engaged full time. Engagement systems can be manually controlled by the operator or can be automatically controlled to engage and disengage the front drive axle. Typically automatic control systems utilize electronic controllers that monitor front and rear axle speeds. When the rotational speed of both the front and rear axles are within a certain range, the controller automatically initiates a shift to engage the front axle.
This automatic controlled engagement and disengagement of the front axle is typically initiated independently from the ground conditions. Thus, engagement of the front axle may not be required or may be poorly timed to maintain vehicle tractive effort. Unnecessary engagement of the front axle results in additional wear of the components, which is undesirable. Further, poorly timed shifts can damage transfer case and axle components resulting in vehicle downtime and increased costs for replacement components.
Thus, it is desirable to have an improved control system for axle engagement that takes into account input and output shaft speeds of the transfer case as well as ground conditions to provide optimal axle engagement shifts.